Aptamers are nucleic acid ligands that bind to their targets, and generally are produced by the SELEX (Systematic Evolution of Ligands by EXponential enrichment) method (Patent Document 1, Non-Patent Document 1). In the SELEX method, a plurality of rounds of selection process are performed, each of which includes the step of bringing a target into contact with an RNA library containing random sequences and the step of amplifying RNA bound to the target. Thus, sequences that can bind to the target are selected from an initial RNA library as desired aptamers.
Although an RNA pool is obtained in every round, the proportion of the desired aptamers in the pool of each round does not increase constantly as the round proceeds. Thus, the number of rounds generally is set based on the empirical rule of experimenters. However, if the number of rounds is set based on the empirical rule, the above-described selection process may be repeated even though a pool in which the desired aptamers are enriched is obtained already, for example. Also, for example, by repeating the above-described selection process a plurality of times, variations of RNAs contained in a pool are reduced. As a result, it appears that RNAs contained in the pool converge to desired aptamers (hereinafter this also is referred to as “enrichment”). However, there may be a case where the desired aptamers actually are not enriched in the pool of the final round. Thus, it is unknown how many rounds need to be performed in order to enrich the desired aptamers in the pool. Thus, from a practical standpoint, checking by a wet lab experiment is necessary, such as checking a pool of each round, checking the binding of RNAs taken as samples from each pool to the target, or checking the proportion of the RNA with verified bonding property to the target in the pool. As a result, there is a problem in that the SELEX method is not effective in terms of labor, time, and cost.